It is known that exposure of human or animal tissue to ionising radiation will kill the cells thus exposed. This finds application in the treatment of pathological cells, for example. In order to treat tumours deep within the body of the patient, the radiation must however penetrate the healthy tissue in order to irradiate and destroy the pathological cells. In conventional radiation therapy, large volumes of healthy tissue can thus be exposed to harmful doses of radiation, resulting in prolonged recovery periods for the patient. It is, therefore, desirable to design a device for treating a patient with ionising radiation and treatment protocols so as to expose the pathological tissue to a dose of radiation which will result in the death of those cells, whilst keeping the exposure of healthy tissue to a minimum.
Several methods have previously been employed to achieve the desired pathological cell-destroying exposure whilst keeping the exposure of healthy cells to a minimum. Many methods work by directing radiation at a tumour from a number of directions, either simultaneously from multiple sources or multiple exposures from a single source. The intensity of radiation emanating from each direction is therefore less than would be required to actually destroy cells (although still sufficient to damage the cells), but where the radiation beams from the multiple directions converge, the intensity of radiation is sufficient to deliver a therapeutic dose. By providing radiation from multiple directions, the amount of radiation delivered to surrounding healthy cells can be minimized.
The shape of the beam varies. For single-source devices, cone beams centred on the isocentre are common, while fan beams are also employed (for example as shown in U.S. Pat. No. 5,317,616).
Of course it is also important that the radiation should be accurately targeted on the region that requires treatment. For this reason, patients are required to remain still for the duration of the therapy session, to minimize the risk of damage to healthy tissue surrounding the target region. However, some movement is inevitable, e.g. through breathing, or other involuntary movements.
To overcome this problem, it is known to integrate an image acquisition system with the radiotherapy apparatus, to provide real-time imaging of the region and ensure that the radiation emitted by the radiotherapy apparatus tracks any movement of the patient. However, the choice of imaging system is in general limited by the radiotherapy apparatus in which it is installed, and in particular by the geometry. For example, magnetic resonance imaging (MRI) systems require magnetic coils to be placed around the patient. However, these coils will act to block therapeutic radiation from reaching the patient.
What is required is an integrated radiotherapy system that delivers high-quality in both the imaging and treatment of a patient.